The repainting of old highway stripes or markings requires accurate registration of the application of the new paint with that of the old marking pattern. In the past, even experienced paint machine operators found that when using machines having manually attended prior art control methods, higher rates of travel made it increasingly difficult if not impossible to paint the new markings at precisely the desired location on the road surface. Clearly, some method of automatic control was needed.
Heretofore, many attempts were made to optically detect the presence of the old marking pattern and to use the output of the detector to automatically switch the paint gun on when the detector sensed the brighter paint line against its background, and to switch the gun off when the detector no longer sensed the presence of the brighter surface. Such attempts were less than satisfactory because all such prior art detectors relied upon the supposed contrast of the reflected incident light from the surface under observation as compared with that of an assumed threshold background light level representing the absence of the paint line. The methods were lacking in reliability in spite of the chosen sources of illumination, whether natural daylight or that provided by strong incandescent lamps.
The amount of brightness differences between a painted line and that of the background surrounding it will vary greatly depending upon the color of the pavement, wear of the paint line, and presence of other discolorations that could activate the prior art detection methods. These earlier methods were also seriously compromised when the pavement had varying ambient illumination, such as when working in areas having alternate sunlight and shade in the path of travel or when the base pavement material changed in adjacent areas, as from concrete to asphalt, for example.
Even when methods were augmented by techniques of color filtering of the reflected light from the surface under observation to give preference to that of the color of the painted marking, such prior art methods were seriously lacking in reliability. This was due to imperfect ability of the prior art detectors to give preference to the reflected brightness of faded or worn painted surfaces as compared with those of adjacent areas of the pavement having various surface characteristics with similar color values and brightness levels.
Because the prior detection methods were not dependable under less than ideal operating conditions, they seriously hampered the successful use of fully automatic guidance methods for positioning the application of new paint directly over the old paint marking patterns. Under those circumstances, a miscue from the detector would cause the paint to be sprayed in the wrong place, a defect not easily remedied after the fact.
Another deficiency of prior art methods of paint line detection was that they could not be made to recognize the presence of black paint on the pavement. It is not universally appreciated that on many roads and highways a great deal of black paint is also used in conjunction with the application of the more familiar yellow or white lane and centerline markings. Black paint is used to give additional contrast for higher visibility, especially on concrete road surfaces where the brightness of the light colored concrete roadway would tend to subdue the visual effectiveness of the brightness of the painted markings. It is also frequently applied in alternate sequence to the normal skipline color markings of roadway centerlines between double solid lines in no-passing zones, and to enhance the edgelines adjacent to the medians separating traffic flowing in opposite directions. The prior art brightness contrast detection methods were of little value as a reliable means for detecting presence of a paint color (e.g. black) that was at the outset intentionally placed on the pavement to be obscure.
With the expansion of highway and road systems a serious need arose for an improved marking system capable of overcoming the limitations and deficiencies of prior systems and also one that was preferably automatic so as to increase the speed of applying roadway markings. Automation of the repainting process involved the satisfactory solution of several practical problems. It was necessary for the control to sense the beginning of the stripe to be repainted, to sense the line continuation and to sense the end of the line marking when the sensing means was used to control paint guns. In addition, it was also necessary for the sensing control to anticipate each of these events in such a fashion that the paint gun turn-on and turn-off delays would not cause the paint application to be late thereby causing the paint to be applied after the paint machine had travelled beyond the proper target locations that initiated the action.
Furthermore, it was also essential that an improved detection method provide enough lead time before the responding paint gun operates in order to allow for the unavoidable operating delay of the gun and for the operating delay time of any positioning mechanism that might be used.
For a more fully automatic control the paint gun package must be capable of moving laterally from side to side to correct for any misalignment due to imperfect positioning of the spray gun in order for the paint application to track the old longitudinal paint markings. This lateral motion must be done quickly so that any lack of alignment will be fully corrected by the time the paint gun arrives at the target location to prevent painting a "hook" at the start of the repainted stripe. The response time of any positioning servomechanism used to position the gun package must be fast enough to satisfy this requirement. For example, it may be called upon to move a gun package mass weighing up to perhaps 500 pounds, and to laterally move such a load 2 inches in as little as 20 milliseconds when travelling at turnpike speeds. Many larger traffic paint marking machines of the prior art have employed gun positioning methods, usually operated by a manually controlled steering wheel. Both mechanical rack and pinion gear methods, as well as hydraulically assisted steering arrangements have been used. A high speed fully automatic control method must include similar positioning elements within a servo-control mechanism, and the sizing of the servo elements such as hydraulic system pressures, piston stroke and diameter and fluid flow rates, for example, must provide for quick response, as described previously.
A sensing method for guiding the positioning servo must have the ability to provide left-right positioning error signals with reference to the centerline of the old paint pattern as the pavement is viewed by the detector at the target area. Heretofore attempts were made to employ television cameras for scanning the area under inspection in order to derive an error signal from the scanned scene television signal. Although the scanning technique provided left-right positioning error signals, the television camera has proved to be no better at discriminating against undesired pickup signals than other prior art optical detection methods, for the same reasons previously discussed.